It is a little bit of a fuzzy boundary, as it varies with metallicity, but it works well enough that the astronomical community has largely agreed on it as the dividing line between gas giants and brown dwarfs.

Quotesteeljaw354 ()

Whats your defintion of a planet?

Well, I've been talking about my views on it throughout the whole thread, but here's a quick summary:

I call an object a planet if it meets the following criteria:

1) It is massive enough to assume a shape by hydro-static equilibrium. (It is round, not potato-shaped.)2) It is not so massive that it is undergoing nuclear fusion in its interior. (It is not a star.)3) It is gravitationally bound to a star. (It orbits a star.)4) It dynamically rules its orbital region. Equivalently, it is unique within its orbital region, rather than belonging to a population of similar bodies.

Notice that 1) provides the lower limit, based on bulk characteristics that arise naturally from principles of physics. When solid astrophysical objects become sufficiently massive, their own gravitation becomes important to describing their overall shape. They collapse into spheres, or more generally oblate spheroids (due to their rotation) or triaxial ellipsoids (by tidal deformation). In all these cases, hydrostatic equilibrium is fundamental to describing the lowest order of relief. For a good primer on this topic, I can recommend Planetary Surface Processes by H. Jay Melosh, and there is also very nice derivation and discussion of "the potato radius" available here.

2) acts as an upper limit, separating the largest gas giants from the smallest stars or brown dwarfs. The fundamental distinction is whether there are any appreciable nuclear fusion processes occurring, which is largely determined by the mass.

3) is a choice of setting. Classically, planets are "wandering stars". They "wander" from night to night relative to the background stars, because they are close to Earth relative to the stars, a result of sharing a common origin and being bound to the same star. This is where the IAU definition holds only within our solar system. I prefer to generalize to apply in any star system. I.e. a planet does not have to orbit "our" Sun, it can orbit any sun, or system of suns. We can call it an exoplanet if we want to specify that it is outside of our own solar system.

It is important to note that all planets do not necessarily remain bound to their stars. In this case they are called rogue planets, freely wandering through interstellar space. There are thought to be many of these out there.

Another way that 3) might fail is if the object orbits the star indirectly, in which case it may be a moon, or a component of a binary planet, depending on the mass ratio.

Finally, 4) is a statement regarding dynamics. There are a huge number of objects in our solar system that satisfy conditions 1-3. We can, if we like, choose to call them all planets, or we can try to narrow it down. Personally, I prefer having on the order of 10 bodies per system be known as planets, not hundreds or thousands. The question is, can I make and justify that choice through something that is not arbitrary? Yes, I can. I notice that there is a small number of bodies in our solar system that satisfy the first three conditions, yet are also very unique from all the others. I also notice that there is an extremely powerful dynamical process that naturally causes this to happen. I choose to make use of this fact to help me to define what I call a planet.

This is made empirical by such measures as the planetary discriminant, or Stern-Levison Parameter. We see that in our own solar system, there are distinct populations of bodies that are separated by several orders of magnitude through these measures. This means there is something enormously important going on, a physical process that causes a small number of objects to grow to dominate their region of space. That's a result of how planetary accretion and the dynamical evolution of systems work. It naturally separates what we call "planets" from all the other stuff we may call "dwarf planets", "asteroids", etc.

So anything that doesn't orbit a star isn't a planet? (not moons, planemos) Even gas giants?So anything that doesn't rule it's region even if it's earth sized or larger isn't a planet?I'm afraid your a bit to logical on this one. Dwarf planets still have the name "planet in them" to me that kinda means they are "half planets" As in the ones smaller than 2000KM. But we won't list them until we know more about them. Or until we can get better sizes on them.

Heres mine1. Doesn't start fusing2. Is round3. Orbits a star, or is rouge4. Size limit on 2000KM for planet anything below is dwarf planet. Pluto and Eris are planets by my 4 rules for a planet. Anything else under 2000KM is a dwarf planet unless it's a potato asteroid. Jupiter,Saturn rule their orbits,Uranus maybe,Neptune Doesn't it is sitting in a field of asteroids.

Earth has asteroids that cross it's orbit and so does almost every other planet, Mercury doesn't dominate it's orbit and mars doesn't. Examples of the orbital domination test, Failed.

Earth has asteroids that cross it's orbit and so does almost every other planet, Mercury doesn't dominate it's orbit and mars doesn't. Examples of the orbital domination test, Failed.

Domination meaning what there are no other bodies of comparable mass. If Jupiter would have Earth-sized bodies near it's orbit, this configuration will be unstable and they quickly will be ejected away or crushed into Jupiter.

Binary systems such as Earth-Moon is not clear in this definition, as long as trojan planets. Trojans are semi-stable though, and only can exist if mass ratio is great (Jupiter can't have another gas giant as trojan planet, or even Earth-mass planet will be unstable there on long timescales).

Doesn't say "orbital domination" on the IAU one it says "orbital clearing" which certainly isn't in the image I provided. There are no known objects near sedna, it's round orbits the sun, so it's a secondary planet by my defintion. Or "dwarf planet" by my defintion. Orbital clearing by the IAU isn't defined.

Doesn't say "orbital domination" on the IAU one it says "orbital clearing" which certainly isn't in the image I provided.

Why are you debating the IAU's definition when nobody here is advocating it? I think everyone here agrees that the IAU's definition isn't great. To replace the arguments made here with those made by the IAU is not a very honest way to conduct a discussion like this.

So your saying there are 8 planets? When sedna is out there on it's own (as we know) being round and all, not being a star, no bodies anywhere near it's size. Something pushed it out there, rouge planet, maybe Planet X? Anything else that has a "similar" orbit could have been pushed away from where it was from planet x (if it exists) There are currently 2 known sednoids Only 2 and the one smaller than sedna is probably a very tiny rock.

Dwarf planets still have the name "planet in them" to me that kinda means they are "half planets"

Sea lions are not actually lions. A guinea pig is not a pig or from Guinea. English can be weird that way sometimes.

Where did you get the value of 2000km as the cutoff threshold diameter for planethood? Is there any physical reason for it, or is it just a number that "felt right" for subjective reasons? There doesn't seem to be any reason why there'd be a natural gap in the size of solar system objects at that scale.

You've been given links several times now to various methods of objectively measuring an object's orbit-clearing capability, and none of them indicate that Mercury, Earth, Mars or Neptune have failed to clear their orbital neighborhoods. Where are you getting your method of determining orbital neighborhood-clearance? It seems like you're just making subjective calls on that, too.

Unless you have some actual process for determining these things that other people can run through and check the results of for themselves I don't think your definition is very useful. Particularly in the context of Space Engine, were we can travel to innumerable other solar systems and see all sorts of arrangements of objects orbiting stars. If I spin the wheel and go to some random solar system out there, how can I determine whether a body has cleared its orbit using your method in such a way that anyone else can go there and get the same answer from the same data?